Inductively Coupled systems, also known as Near Field Magnetic Induction (NFMI) systems, provide wireless communication or wireless power transmission within a localized magnetic field (the near field) of a transmitter.
The power density of near-field transmissions is highly localized and attenuates or decays off at a rate proportional to the inverse of the range to the sixth power (1/range6) or −60 dB per decade. In contrast, a radiated electromagnetic field decays at −20 dB per decade (1/range2). Coupling efficiency and range may be increased by using tuned transmitters and receivers. Inductively Coupled communication systems provide secure wireless communication between a transmitter and receiver. Example applications of Inductively Coupled communication systems include radio frequency identification (RFID), Near-Field Communication (NFC) and wireless charging systems.
One restriction on the use of Inductively Coupled systems is that metallic objects or surfaces affect the inductance of the receiver or transmitter antennas by channeling some or all of the magnetic field flux away from the receiving antenna. One approach to mitigate this problem uses a ferrite shield as a barrier between the antenna and the metallic object. However, the physical construction of a ferrite shield makes it impractical for many applications. For example, a ferrite barrier may be constructed of a ferrite powder in a flexible matrix material. Increasing the ferrite content limits the flexibility of the barrier and makes it brittle, so a minimum thickness of material is required. Ferrite shield materials typically have a thickness of 0.1 mm or more and are not uniform enough to limit the magnetic field interaction or to provide a consistent structure to guide the magnetic flux around the windings of the antennas.
It would be useful to provide an effective barrier between an antenna of an inductively coupled communication system and a metallic object.